I have a 66 396 that I have always had trouble with but wanted to keep in my car just due to saying numbers matching. I rebuild that thing bored big cam roller rockers and so on. So here my issue it's a as I told a late 65 early 66 motor and since I have all the new stuff on it I would like to use. The motor ran pretty good once installed but then I ran up a stump with sickness, jobs and just life I let it sit in my garage and thought I drained the water out of the rad, and block but that winter even in my gargle got could enough to push 2freeze plugs out. So to present time got fired back up but it seemed to turn slow so I got new starter still slow, so pulled dip stick and I have water. So here's my question would yaw just start all over with a complete setup such as a newer big block which my stuff I have now will not work or find a block and put my stuff in? Or another thought get my block tested before anything and see if it is cracked or not?

Test it. You may even see noticeable damage inside the block with the naked eye if it's cracked. I've seen one that had a golf ball size hole in there up between the camshaft area and bottom of cyl's from freezing and bustin'.

Also, did you use a camshaft that was compatible with the early block? If it's '66 the rear journal on the camshaft has to be grooved.

Its up to you. 396 blocks are kind of cheap since everyone wants a 454 block now. So it may worth rebuilding what you have. Have yours checked and see what failed. But that being said, the cost of a rebuild for a 396 and 454 is the same.

but since it is the numbers block, do have it evaluated. if it has minor flaws, even some cracks, some super machinists here may be able to save your block for you.
WOLFPLACE in N California is one such guy. He may know someone local to you that can fix. Again, all depends.

once fixed, store the born-with block (so you can preserve it for posterity and not feel bad about blowing or wearing out the next engine).

454 block, your crank and rods, new pistons for 427 and you have a 427
or do a 454
or buy a 496 built